How do we know that Mary knew she had a choice in whether or not to be Jesus's Mother? I'm not trying to be obtuse or disingenuous, I'm honestly wondering. It was an Angel of the Lord, after all, who made the announcement, and it was not couched as a request.

Answer by Catholic Answers on 1/2/2014:

Cecilia--

Keep in mind that the Gospels are not court transcripts, they are historical documents written in a time when writers did not have our own sense of what constitutes accuracy. I am not saying the Gospels are not accurate; they are, but they are accurate according to the style in which their authors wrote.

Basically, what this means is that St. Luke's account was not concerned so much with the angel's exact words to Mary and her response, but with the fact that the angel was announcing the Incarnation and with Mary's Yes to that announcement. What matters to St. Luke is not that Mary had a choice to make, but that the choice she made was to say yes to God's plan.

It is only later, when theologians began to unpack Christian doctrines such as human free will that it became clear that there was a choice to be made by Mary at the Incarnation because the implications of there not having been a choice would have been dreadful. For example, if we say that Mary had no choice, then we would have to say that the Incarnation was a form of divine rape, similar to the some of the mythological conception stories. Since it is absolutely impossible for the one true God to have done such a thing, we must therefore conclude that Mary had free choice in the matter.